A black hole is a volume from which photons, or any matter, can not escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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Can light change the shape of a black hole?

If light orbits a black hole without ever getting closer or further from it, and the black hole pulls on the light from all directions nearly simultaneously, does the black hole extend at its equator? ...
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How to find tetrads corresponding to a given metric?

In General Relativity, sometimes doing calculations with a set of orthonormal tetrads $h^{a}_{\mu}$ where $h^{a}_{\mu}$$h^{b}_{\nu}$$\eta_{ab}=g_{\mu\nu}$ for some metric $g_{\mu\nu}$ is easier ...
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Why do stars having mass less than 3 X mass of sun do not collapse into a black hole

Why do stars having mass less than 3 X mass of sun do not collapse into black holes?? Is there a way to find out the radius of a black hole if we know the mass of the parent star and vice versa??
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What is black hole spin?

First of, congrats to the people at LIGO. In this article, the BBC notes that the latest LIGO results show that a new black hole was formed with a spin of $0.2$ (dimensionless number). What exactly ...
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How does the mass of a black hole varies?

Is there any change of the mass of a black hole if time passes ? As it absorb any mass near it, will the mass of the black hole gets increased, or as it radiates energy, will the mass of it gets ...
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Why does Ligo's second detection of gravitational waves and a black hole merger look absolutely nothing like the first? [closed]

Why does Ligo's reported second detection of gravitational waves and a black hole merger look absolutely nothing like the first detection announced in Februaray? Here is the data from the first LIGO ...
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AdS Black holes

How is the mass of the black hole defined in a asymptotcally AdS solution of the black hole? How can I find it? Beacause in asymptotcally flat solution I can read it from the $g_{tt}$ component of the ...
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Black hole simulation

NASA published a computer generated black hole image. In the image you can clearly see the event horizon and the light of background stars graduating from "smeared" to normal. However, between the ...
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Object traveling towards a black hole close to the speed of light [duplicate]

Fluff: I will start by saying my knowledge of physics lacking to say the least. I am asking this question purely out of curiosity. The question: If an object was traveling close to (or at (if it ...
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If the speed of light is constant, why can't it escape a black hole?

When speed is the path traveled in a given time and the path is constant, as it is for $c$, why can't light escape a black hole? It may take a long time to happen but shouldn't there be some light ...
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Effect of black hole mergers on life [closed]

Would the gravitational energy released in the explosion affect life in any way if it were on a planet close by?
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Faster than light travel…? [duplicate]

So - Black holes possess a gravitational force so great it can pull anything in. Including light. So placing these side by side, If I were to shoot a laser pointer out of a cannon at a black hole, ...
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GR Verification for a Charged Black Hole

For a charged ($Q$) rotating ($L$) mass ($M$), the Kerr-Newman equations give the angular deflection of light. Has there been observational verification (I would prefer to use only the angular ...
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Does binary black holes evaporate slower because they compete for attracting virtual particles?

Is it true that if 2 black holes are near each other, such as binary black holes, will evaporate slower than when they are far apart, because they are competing for attracting virtual particles each ...
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Merging black holes makes them less dense, so

According to What is exactly the density of a black hole and how can it be calculated? (more specifically, John's answer here made me think: if you merge a whole load of chunks of an element heavier ...
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Does space itself fall into a black hole? [duplicate]

Long time ago I heard someone say that it is space itself that falls into a black hole. Yesterday I saw a little animation that suggested the same (although I´m not sure, because the person who put ...
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Must the fine structure constant be an irrational number? [closed]

This question concerns a conjecture on the nature of $\alpha = e^2/4\pi\epsilon\hbar c$ in light of black hole holes. Let me consider the Reissnor-Nordstrom metric for a charged black hole $$ ds^2 = \...
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Is Hawking radiation really the same as Unruh radiation

I read that Hawking radiation is the same as Unruh radiation. However, there seems to be a paradox here. If you have an extreme black hole (say with maximum charge), then it has temperature 0 and ...
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What is the singularity of an actual collapsing black hole?

In most general relativity texts, the singularity is treated as a point removed from the manifold, to avoid having to deal with the infinite curvature of the Ricci scalar. But in the case of a more ...
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Charge without charge and non-traversable wormholes

My question concerns the theory proposed in this classic paper by Misner and Wheeler. In the paper, the authors propose the idea of "charge without charge"--namely, that positive and negative ...
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Low density black holes and singularity

According to wikipedia "... the average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the ...
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BTZ Black holes

I computed the two point function for two scalar fields in BTZ black hole that is defined as a local $AdS_3$ space time with discrete identification, as defined in many papers. Referring to this paper,...
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Orbits around the Photon sphere of a black hole (Schwarzschild coordinates)

This is a follow-up question to the answer given at What is the exact gravitational force between two masses including relativistic effects?. Unfortunately the author hasn't been online for a few ...
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Could gravitational waves have a large entropy in quantum gravity?

The question is broad, and we still don't know what is quantum gravity. But let me be more specific. In the question (on this site, see below) on whether gravitational waves have entropy the answers ...
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Could planets survive their star becoming a black hole? [duplicate]

Would the supernova responsible for its formation destroy them. The question becomes one of how destructive the supernova event is to its own solar system - does it completely evaporate all planets up ...
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Are the quasinormal modes scalar quantities?

I am studying the so-called quasinormal modes (QNMs) in the context of the AdS/CFT correspondence and I got stuck. For instance, if I choose a weird patch of coordinates for the, say, AdS5-...
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Would a particle the size of a neutron, if it had enough mass, collapse into a blackhole?

For example, a neutron is a particle that occupies a certain volume. If you pack enough mass into that volume, it would collapse into a black hole (I assume there is not enough mass now). At least if ...
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Kerr black hole horizons and infinite redshift surfaces

In the Kerr black holes we have infinite-redshift surfaces (where a infalling body is still according to the asymptotic observer) and event horizons (the escape velocity becomes greater than the speed ...
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Two Black Holes held stationary by EM forces

If two black holes with large enough mass (so that the tidal forces are minimal and the intersection is large) that are held apart by like charges (saddle point stability). Imagine the black holes in ...
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Torsion in kerr black holes

In General Relativity, we generally assume that the derivative operator is torsion-free, i.e., second covariant derivatives commute on functions. However, in Kerr black holes, spacetime is dragged (...
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Do the energies of cosmic rays approach infinite at the event horizon of a black hole?

Let's assume an observer orbits close to a black hole, he is not alone, massive cosmic rays, like electrons and protons and other kind of space dust comes from the outer space and may hit him. Since ...
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Schwarzschild metric black hole

Schwarzschild metric solution presents two singularities. An apparent one at $r=2GM$ and a real one at $r=0$. It is known that everything freezes at the event horizon from an outside observer point of ...
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Can we “see” into a black hole using gravity?

I believe the "no hair" theorem means all black holes settle down into a state only determined by a few parameters, typically listed as mass, charge and angular momentum. But I don't think they can ...
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Can objects exit a black hole?

I read an article claiming that an object had been observed to be ejected from a black hole. This was the newest article I could find published on it but they still seem uncertain what the object was ...
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Will two entangled particles changing Synchronously while one is in a black hole and the other is not?

1.If you change the attribute of one entangled particle then the other one changes too and instantaneously. 2.The time will go slower in (or near to) a black hole compare to our normal place like on ...
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Collapse of two large black holes in AdS

In $4d$ flat space, two black holes of mass $M$ can collapse to form another one of (roughly) mass $2M$. This process is spontaneous, as reflected by the fact that the black hole entropy $S=M^2$ ...
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Can black holes grow via accretion of dark matter particles?

I'm assuming that the answer to the question in the title is a resounding yes. Since Baryonic matter and dark matter interact via gravitational forces. If this is the case how is information not lost ...
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Would a black hole cast a shadow? [duplicate]

Okay, so I've been wondering and this is a simple question and I cannot find an answer anywhere. Would a blackhole the size of the Moon (event horizon) black out the Sun in a "Lunar eclipse"? My ...
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'Hovering' light rays on the edge of a black hole

According to Prof. Hawking, light rays will 'hover' on the edge of a black hole. If this is true, and the light 'stops' on the edge, how can the electric/magnetic fields which, constitute the light, ...
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How much energy can be extracted by lowering something into a black hole? [duplicate]

If an object is in orbit around a star, the object has gravitational potential energy that could possibly be extracted. For example, when we perform gravitational slingshots around Jupiter, our ...
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What happens to objects sucked into a black hole after the black hole evaporates away?

Suppose an object falls into a black hole that's so massive that it wouldn't get torn apart at the event horizon. What happens to it after the black hole evaporates away? According to the theory ...
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Is there such thing as imaginary time dilation?

When I was doing research on General Relativity, I found Einstein's equation for Gravitational Time Dilation. I discovered that when you plugged in a large enough value for $M$ (around $10^{19}$ ...
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Can a High Enough Temperature Create a Black Hole?

My very basic understanding of GR leads me to think that if a substance has a high enough temperature, it can transform into a black hole without a mass required to create a black hole. The equation ...
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Universe Expanding

Quick question that I was just hoping to get some clarification on. Current theories state that the universe is expanding; with those planets farther away moving away from us at a faster rate. Is it ...
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Can the “black hole bomb” instability be made to work with small discrete mirrors instead of a spherical mirror?

Is it possible for the so-called "black hole bomb" instability to work with small discrete, orbiting mirrors instead of one spherical mirror? This would be far more practical for a hypothetical ...
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Luminosity of an accretion disk?

With reference to Black holes in particular, how can you approximate the luminosity of an accretion disk? It is possible to quantify the temperature at a given point, but as the disk is not a black ...
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Spinning black hole laser?

Is a laser that uses a rotating black hole's ergosphere as the gain medium possible? The idea is that the coherent light feeds off of the black hole's rotation. Assume that the black hole is ...
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Rainbow Black Hole [closed]

In the photon sphere of a black hole the photons are being trapped in orbit, pulled in and trajectories changed. From a safe distance should we see color from the entire spectrum if light in the ...
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What would happen if a black hole merged with a white hole of the same size?

I learned about white holes a while back, this was the first question that came to mind.
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Current rate of supermassive black hole merges

I think, it is probably not a very common event, and if it happens, it can be maybe one of the ...most interesting events of the Universe. Considering that the merge of two stellar blackholes were ...